Laminated package material, method for manufacturing the same, and a package

ABSTRACT

The present invention relates to a laminated packaging material suitable for use especially in heat-sealable packages for liquid food, such as milk, cream and juice cartons, and to a manufacturing method of the material, and to a food package manufactured of the material. The material comprises a core of paper or cardboard, and oxygen and aroma proof barrier layers of EVOH and polyamide. The EVOH layer and the polyamide layer are joined together directly without any intermediate adhesive layer, and the barrier layers and the core are interposed between polymeric heat-sealing layers on both sides of the material. The material is manufactured by extrusion so that at least the materials of the said barrier layers are placed against each other in a molten state and forwarded to the core in one web.

This application is a division of 09/214,787 filed Mar. 12, 1999, nowU.S. Pat. No. 6,383,582, which is a 371 of PCT/FI97/00407 filed Jun. 25,1997.

BACKGROUND OF THE INVENTION

The present invention relates to a laminated packaging material which isespecially suitable for heat-sealable packages for liquid food, andwhich comprises a core of paper or cardboard, and oxygen and aroma proofbarrier layers of ethylene-vinyl alcohol copolymer (EVOH) and polyamide(PA). Further, the invention relates to a manufacturing method of thesame and to a sealed package for liquid food manufactured of thematerial, e.g. a milk or juice carton.

Laminated packaging materials used for liquid food packages and based onpaper or cardboard are usually provided with one or more barrier layers,the purpose of which is to prevent the contamination of the packedproduct due to oxidation, and to preserve the product's taste andvitamins contained by it. Of polymeric materials, EVOH is known to haveexcellent barrier properties, but also polyamide has been used in liquidpackage cardboards because of its good barrier properties.

An example of the use of polyamide in a liquid package cardboard isdescribed in the publication print FI 86610 and, correspondingly, theuse of EVOH is described, for example, in the publication print FI89567. In accordance with both the prints, the packaging material hasbeen laminated on both sides with an external heat-sealing layer of LDPE(low-density polyethylene) so that the cardboard layer and the barrierlayer of polyamide or EVOH is interposed between the heat-sealinglayers. In the patent publication U.S. Pat. No. 4,701,360, there areknown laminated packaging materials for liquid packages which may,besides an EVOH layer, include another oxygen proof barrier layer e.g.of polyamide. According to the publication, the EVOH layer is situatedon the surface of the packaging material so that it also acts as aheat-sealing layer. In this case, EVOH is in contact with the packedliquid, which is a poorer solution with regard to the barrier propertiesof the material than the one in which the EVOH layer would be protectedby a separate heat-sealing layer.

In the EP publication 0 318 771 there is disclosed a packaging laminatecomprising EVOH and polyamide layers in combination with an aluminumfoil. According to this publication the aluminum foil constitutes themain gas barrier of the laminate, and this barrier has been enhanced byan EVOH layer in order to compensate for small cracks that might occurin the foil. An optional polyamide (nylon) layer may be included toabsorb moisture penetrating through said cracks. According to theteachings this layer is in general not necessary but preferred when thelaminate is in danger of being subjected to strong external stresses,that is, in danger of developing cracks. If a polyamide layer isincluded it would have a thickness of 15-25 μm, preferably 20 μm. Thealuminum foil in such a laminate forms an excellent oxygen barrier, buta drawback is the high cost of the foil.

The problem with EVOH in cardboards for liquid packages is that it ismechanically weak. For example, upon manufacture of carton-shapedpackages, the material has to be folded, which easily causes cracks inthe EVOH layer weakening its barrier properties. In addition, the EVOHlayer tends to form holes in the area of its joints when the package isheat-sealed. This is caused by steam pressure which tries to escape fromthe cardboard layer because of heat and which EVOH cannot retain. Inthis respect, polyamide is a better material than EVOH, because it isless vulnerable to cracking upon folding, and it can also absorbmoisture released from the cardboard. Because the barrier properties ofpolyamide are not equal to those of EVOH, it cannot as such replaceEVOH, but it has been tried to solve these problems by combining EVOHand polyamide in a packaging material. Thus, the publication print FI96752 discloses a method for solving the problem related with theformation of holes by a suitable choice of material for the barrierlayer, one alternative being a compound of EVOH and polyamide. Thepurpose is to retain the viscosity and strength of the barrier layermaterial in the heat-sealing temperature. Also the patent publicationU.S. Pat. No. 4,977,004 disclosing a food package cardboard comprisingtwo separate EVOH layers recommends the compounding of EVOH andpolyamide to improve the viscosity of the layer. However, a drawback ofthese solutions is that the barrier properties of EVOH suffer due to thecompounded polyamide. The best way to make the material oxygen and aromaproof is to include a continuous, substantially clean EVOH layer in thepackaging material. In addition, it has been discovered that thecompound of EVOH and polyamide is more difficult to extrude than purepolymeric materials, mainly due to the tendency of the compound to gel.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a solution forcombining EVOH and polyamide especially in a laminated packagingmaterial for liquid food without the above-mentioned drawbacks. Thus, itis the object of the invention to prevent the formation of holes in thelayer containing EVOH, and, simultaneously, to keep the amount ofpolymer in the layers as low as possible. The laminated packagingmaterial of the invention is characterized in that the layer ofethylene-vinyl alcohol copolymer (EVOH) and that of polyamide (PA) arejoined together without a layer of adhesive material inbetween, and thatthe material comprises a polymeric heat-sealing layer on both sides sothat the core of paper or cardboard and the said barrier layers areinterposed between the heat-sealing layers.

DETAILED DESCRIPTION OF THE INVENTION

Thus, the present invention is based on the observation that theexcellent barrier properties of EVOH and the viscosity and absence ofholes in polyamide may be combined, without deteriorating eitherproperty, by joining the barrier layers of the said materials directlyto each other without an intermediate adhesive layer, such as a Surlynlayer, which has typically been used to adhere different layers inliquid package cardboards. The good adhesion of the EVOH and polyamidelayers observed in accordance with the present invention is supposedlydue partly to a chemical reaction between polymers and partly topolarity, i.e. the setting of the negatively charged EVOH and thepositively charged polyamide. The absence of the adhesive layer reducesthe number of layers compared to other conventional multi-layerstructures, which means the extrusion process becomes simpler and causesa saving in materials. The saving in materials and the lighter structuremay also be achieved because the necessary barrier properties and theabsence of holes may be provided with smaller amounts of EVOH andpolyamide than before. In general, the laminated polymeric layers ofcardboard potentially have a negative effect on the smell and taste of aproduct, and, for this reason, the invention serves the general goal tokeep the amount of polymers in a packaging material as low as possible.The packaging material of the present invention can also endure thesterilization processes included in aseptic packaging.

In accordance with the present invention, it is characteristic of EVOHand polyamide that they can be joined together or to a core of paper orcardboard in the packaging material without an intermediate adhesive. Anadhesive layer may instead be necessary between the barrier layers andthe heat-sealing layer on the material surface. The heat-sealing layerson both sides of the material may advantageously be made of polyolefin,such as LDPE. For example, Surlyn or LDPE modified with maleicanhydride, may be used as adhesive.

In case the barrier layers are arranged in the packaging material sothat the polyamide layer is closer to the paper or cardboard core, thepolyamide protects the EVOH layer by receiving and absorbing moisturereleased from the cardboard layer upon heat-sealing. The EVOH layer isprotected against the formation of holes, and it retains itsimpermeability to oxygen and aromas in the joints. However, it is alsopossible to place the EVOH layer against the paper or cardboard core,which is less advantageous for the tightness of the layers, but which,on the other hand, has the advantage of the excellent adhesion of EVOHto paper or cardboard. In this case, no limits have to be set for thequality of paper or cardboard used in the packaging material.

For the barrier properties, the sufficient amount of polyamide in thepackaging material may be about 1-8 g/m², preferably 2-5 g/m², and theamount of EVOH likewise about 1-8 g/m², preferably about 2-5 g/m². Inknown packaging materials with a barrier layer either of EVOH orpolyamide, the typical amounts have been about 6-8 g EVOH/m² and 8-10 gPA/m². Thus, the separate, opposite EVOH and polymer layers of thepresent invention do not increase the amount of polymer required by thebarrier layers in the packaging material, but, rather on the contrary,they may be able to reduce it. The amount of polymer in the heat-sealinglayer may be about 5-50 g/m² on both sides of the material.

The method of the present invention for manufacturing the said laminatedpackaging material, in which polymeric laminate layers are placed on acore of paper or cardboard by extrusion, is characterized in that EVOHand polyamide are placed against each other as molten webs, whereafterthey are forwarded to the core in a combined web. The barrier materialsthen have time to become effectively amalgamated before they meet thepaper or cardboard, and, thereafter, solidify.

A liquid food package of the present invention, which is manufacturedfrom the said packaging material by heat-sealing, is typically a sealedcarton mainly of rectangular shape, and it may contain, for example,milk, cream, sour milk, yogurt, juice, or wine. However, it may bementioned that the polymeric barrier layers do not act as an oxygen oraroma barrier in an unopened package only, but they also help topreserve the product even after the package has been opened.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is next described in more detail with the help ofembodiment examples and reference tests, and referring to the encloseddrawings in which:

FIG. 1 is a schematic view of a laminated packaging material accordingto the present invention;

FIG. 2 is a schematic view of another laminated packaging material ofthe present invention; and

FIG. 3 describes extrusion of polymeric layers in the material and theirplacement on the cardboard.

DETAILED DESCRIPTION OF DRAWINGS

The second surface layer 2 of the packaging material 1 of the presentinvention, forming the outer surface of the package to be manufacturedand acting as a heat-sealing layer in the package, is made oflow-density polyethylene (LDPE). Next, there is the core 3 manufacturedof cardboard, e.g. chemi-thermo-mechanical pulp (CTMP). Thereafter,there are the oxygen and aroma proof barrier layers 4, 5, which arejoined together and to the cardboard core 3 without intermediateadhesive layers. In the embodiment of FIG. 1, the barrier layer 4against the core 3 is polyamide (PA), and the second barrier layer 5adhered to the polyamide layer is ethylene-vinyl alcohol copolymer(EVOH). The EVOH layer 5 is followed by an adhesive layer 6 of, forexample, LDPE modified with maleic anhydride, the purpose of which is tobind the EVOH layer to the second surface layer 7 of the material, whichis LDPE. Upon binding, the said surface layer 7 acts as a heat-sealinglayer joining the second surface layer 2, and it forms an inner surfacewhich is in contact with the liquid in the finished package.

The package material 1 of FIG. 2 is similar to that in FIG. 1, besidesthe order of barrier layers 4, 5 of EVOH and polyamide. In FIG. 2, anEVOH layer 5 is placed against the cardboard core 3, the followingpolyamide layer 4 being joined to the heat-sealable LDPE layer 7 formingthe inner surface of the package by an adhesive layer 6.

In FIG. 3, there is schematically shown the manufacture of packagingmaterial 1 according to FIG. 1, which is carried out by extruding thepolymeric layers 2, 4-7 to a moving cardboard web 3 forming the materialcore, the extrusion process being controlled by rollers 8. Beforecoating, the cardboard web 3 is subjected to corona treatment 9. Thefour superimposed polymeric layers 4-7 to be placed on the cardboard areeach led from their own containers 10-13, from where the polymers areguided to a nozzle 14 which amalgamates the molten polymers to eachother in layers and feeds the polymers to the cardboard 3 in onecombined web 15. Extrusion of a polymeric heat-sealing layer 2 to beplaced on the opposite side of the cardboard web 3, not shown in FIG. 3,may take place substantially in a similar way. The amounts of polymer tobe extruded may be e.g. about 2-10 g/m² in the barrier layer 4, about2-10 g/m² in the barrier layer 5, about 3-10 g/m² in the adhesive layer6, about 10-50 g/m² in the heat-sealing layer 2 opposite the material,which is not shown in FIG. 3. The weight of the cardboard core 3 may beabout 200-400 g/m².

Reference tests

A laminated packaging material according to the invention wasmanufactured by forming it, in accordance with FIG. 1, from a LDPE layer2 with a weight of 20 g/m², a cardboard layer 3, a polyamide layer 4with a weight of 5 g/m², an EVOH layer 5 with a weight of 5 g/m², anadhesive layer 6 with a weight of 5 g/m², and a LDPE layer 7 with aweight of 41 g/m². This was compared to three reference materials, thefirst of which (reference material 1) comprised a LDPE layer with aweight of 14 g/m², a cardboard layer, an EVOH layer with a weight of 5g/m², an adhesive layer with a weight of 6 g/m², and a LDPE layer with aweight of 45 g/m²; the second (reference material 2) comprised a LDPElayer with a weight of 20 g/m², a cardboard layer, a polyamide layerwith a weight of 5 g/m², an adhesive layer with a weight of 6 g/m², anda LDPE layer with a weight of 41 g/m²; and the third (reference material3) comprised a LDPE layer with a weight of 20 g/m², a cardboard layer, abarrier layer including as a compound 47.5% EVOH, 47.5% polyamide, and5% ethylene-methyl acrylate copolymer, with a weight of 5 g/m², anadhesive layer with a weight of 6 g/m², and a LDPE layer with a weightof 41 g/m².

Of the said materials, the oxygen density was first determined bymeasuring the amount of oxygen which penetrated the material at 23° C.per square meter of the surface in 24 hours as an average of penetrationfrom 2-4 test specimens. The results are shown in Table 1.

TABLE 1 O₂ penetration Average thickness of the Material (cm³/m²/24 h)material (μm) Invention 2.56 506 Reference material 1 2.87 503 Referencematerial 2 7.0 505 Reference material 3 11.5 512

The vulnerability of the materials to form holes in the heat-sealingprocess was thereafter tested by a so-called bubbling test, in whichhot-air blasting was used. The test was carried out as follows:

The material specimen to be tested was placed to a stand which movedback and forth with a constant speed of 200 mm/s under hot-air blasting.The distance from the blower to the surface of the material specimen was7 mm, and the temperature of the blast air 350° C., and the relativemoisture 100%. The blasting made the plastic to melt. The resistance ofthe material to hole formation could then be determined on the basis ofhow many times the material passed the blower before the first holeswere formed, and how the hole formation developed while the movementcontinued. The hole formation of the material was researched and ratedon a four-step scale which was as follows:

E=undamaged; H=very few holes; V=a few holes; and P=many holes. Theresults are shown in Table 2.

TABLE 2 Number of times passed 1 2 3 4 5 6 7 8 9 10 Invention E E E E EE E E E E Reference material 1 E E E H H V P P Reference material 2 E EE E E H H H H H Reference material 3 E E E E E H H H V V

The results show the packaging material of the present invention to bebetter than the three reference materials. The superiority wasespecially evident in the bubbling test measuring the vulnerability ofthe material to hole formation, in which the material of the inventionlasted 10 passes (5 reciprocating movements) under hot-air blastingwithout hole formation. The test was not continued further, because thecardboard core of the material would have started to become charred.Hole formation was observed in all three reference materials at thelatest after six passes, and especially the reference material 1comprising the mere EVOH layer was totally damaged already after sevenpasses due to abundant hole formation.

It is obvious for those skilled in the art that various embodiments ofthe invention are not limited to the above examples, but they can varywithin the scope of the enclosed claims.

1. A laminated packaging material especially for heat-sealable liquidfood packages, comprising a core of paper or cardboard, and oxygen andaroma proof barrier layers consisting of ethylene-vinyl alcoholcopolymer and polyamide layers, wherein the weight of the polyamidelayer is in the range of 1 to 8 g/m², and wherein the weight of theethylene-vinyl alcohol copolymer layer is in the range of 1 to 8 g/m²;that the layer of ethylene-vinyl alcohol copolymer and the layer ofpolyamide are joined together without an intermediate adhesive layer,and that the material includes polymeric heat-sealing layers on bothsides so that the core and the said barrier layers are situated betweenthe heat-sealing layers; wherein the barrier layers, joined together areplaced directly against the core of paper or cardboard without anintermediate adhesive layer; wherein the barrier layers do not containan aluminum layer; and wherein the barrier layer of ethylene-vinylalcohol copolymer layer is placed directly against the core of paper orcardboard; and the material further comprises an adhesive layer having aweight in the range of 3 to 10 g/m² which is between one of said outerheat-sealing layers and one of said barrier layers.
 2. A packagingmaterial in accordance with claim 1, wherein the heat-sealing layersforming the surface layers of the material are made of polyolefin.
 3. Apackaging material in accordance with claim 2, wherein the polyolefin islow-density polyolefin.
 4. A packaging material in accordance with claim1, wherein the weight of the polyamide layer is 2 to 5 g/m².
 5. Apackaging material in accordance with claim 1, wherein the weight ofeach of the heat-sealing layers is in the range of 5 to 50 g/m².
 6. Amethod for manufacturing a packaging material of claim 1, in which thepolymeric laminate layers are placed on the paper or the cardboard coreby extrusion, wherein the ethylene-vinyl alcohol copolymer and polyamideare placed against each other in molten webs, and thus forwarded to thecore in one, combined layer.
 7. A sealed package for liquid food,wherein the package is manufactured by heat-sealing from a packagingmaterial of claim
 1. 8. A packaging material in accordance with claim 1,wherein the packaging material is a liquid packaging material.
 9. Alaminated packaging material especially for heat-sealable liquid foodpackages, comprising a core of paper or cardboard, and oxygen and aromaproof barrier layers consisting of ethylene-vinyl alcohol copolymer andpolyamide layers, wherein the weight of the polyamide layer is in therange of 1 to 8 g/m², and wherein the weight of the ethylene-vinylalcohol copolymer layer is in the range of 1 to 8 g/m²; that the layerof ethylene-vinyl alcohol copolymer and the layer of polyamide arejoined together without an intermediate adhesive layer, and that thematerial includes polymeric heat-sealing layers on both sides so thatthe core and the said barrier layers are situated between theheat-sealing layers; wherein the polyamide layer is placed directlyagainst the core of the paper or cardboard without an intermediatedadhesive layer, to absorb moisture released from the core and protectthe ethylene-vinyl alcohol copolymer layer against formation of holesupon heat-sealing; the material further comprises an adhesive layer of apolymer other than those of said barrier layers, which is between theethylene-vinyl alcohol copolymer layer and one of said outerheat-sealing layers; and no aluminum layer is present in the material.10. A packaging material in accordance with claim 9, wherein there is anadhesive layer having a weight in the range of 3 to 10 g/m² which isbetween one of said outer heat-sealing layers and one of said barrierlayers.